This is a revised RO1 application originally reviewed by the MDA Review Committee in June, 1997. Elderly individuals have a high rate of both subjective and objective sleep disturbances, and these sleep problems have significant health consequences. Developing effective non-pharmacological treatments for sleep disturbances in the elderly is an important goal. However, behavioral sleep interventions for the elderly should be based on the underlying sleep physiology of the elderly. Current behavioral interventions are based on physiological models of sleep regulation developed in younger adults, which assume a strong circadian pattern of sleep tendency. The elderly may have a different pattern, marked by reduced amplitude of the circadian rhythm in sleep tendency. Thus, behavioral treatments which emphasize a single, consolidated nocturnal sleep period may not be consonant with underlying sleep physiology of the elderly. A controlled laboratory study is crucial in order to define the circadian sleep physiology of the healthy elderly. The major aim of this project is to compare circadian patterns of sleep tendency and subjective sleepiness in healthy elderly and young adults. Secondary aims are: To examine age-related differences in circadian patterns of mood, activation, and psychomotor performance; and to examine the relationships between circadian core body temperature (CBT) rhythms and circadian patterns of sleep tendency, mood, activation, and performance. Subjects will include 20 healthy adults 20-30 years old (10 women and 10 men) and 20 healthy adults 70-85 years old (10 women and 10 men). Subjects will be studied on a schedule of 30 minutes sleep alternating with 60 minutes of wakefulness, which will continue around the clock (i.e., at multiple circadian phases) for 40 cycles (2 1/2 days). This study will test the following specific hypotheses: 1) Compared to young adults, elderly subjects will have a "flattened" circadian pattern of objective and subjective sleep tendency. Specifically, the circadian pattern of sleep tendency in elderly subjects will demonstrate a lower mean level (i.e., less overall sleepiness), reduced amplitude, and an earlier phase. 2) Compared to young adults, elderly subjects will have reduced amplitude in circadian patterns of mood, activation, and psychomotor performance (manual dexterity, four-choice reaction time, and a stop signal task). 3) The timing (phase) of the core body temperature rhythm will correlate with the timing (phase) of circadian rhythms in sleep tendency, mood, activation, and performance in both young and elderly adults. This study will yield important new information regarding sleep physiology and circadian rhythms in the elderly. These data will help us to better understand the widely prevalent sleep problems of elderly individuals and to develop more effective physiologic assessments and behavioral sleep interventions for them.